Calibrating device



Oct. 22, 1946. P. c. GARDINER ET AL CALIBRATING DEVICE Filed Nov. 24, 1941 mooum TING OSt/Ll A Ton TUNED (mm/T l5 l4 CIRCUITS l I Q? FREQUENCY DETECTOR convem-zn mo F/ H I AND AUDIO l |1\ n -AMPLIFIER mpuruza 1. j. 1 :11:: tzll l j I A.V.C. o CRYSTAL AMPLIFIER sc ggk I 22 J 49 -82 Mom/m mva 519 55 T 05611111701? rah/0 0:75am? 19- 2 u 9 F) 3. E 4 44 m /0/ 5 45 z a 5 2: 5: 0 o a 4/ u 226MC RE FREQUENCY 227m CALIBRATION ADJUSTMENT Inventors: Gaul C. Gar diner John E. Maynard;

Patented Oct. 22, 1946 CALIBRATIN G DEVICE Paul C. Gardiner, Scotia, and John E. Maynard,

Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application November 24, 1941, Serial No. 420,230

3 Claims.

Our invention relates to a calibrating device, and more particularly to such a. device for calibrating variably tuned apparatus.

variably tuned amplifier and oscillator circuits operating at ultra high frequencies, for example, of the order of 50 to 500 megacycles, are subject to frequency drift which is generally large with respect to the frequency band Width necessary to translate an ultra high frequency wave through such circuits. It is, accordingly, desirable to tune such circuits more broadly than is necessary to accommodate the frequency band width of.

such ultra high frequency waves, so that the effect of such drift is minimized. For example, in ultra high frequency radio receivers, the receiving circuit may be capable of responding to an ultra high frequency wave anywhere within a band of frequencies much wider than the highest transmitted audio frequency. When thus broadly tuned, the local oscillator in a superheterodyne receiver may drift in frequency by substantial amounts without causing the intermediate frequency to change to a region outside of the intermediate frequency pass band.

It is desirable that such circuits should be so calibrated that, before drift occurs, the frequency of the translated wave is centered in the pass band of the circuit when they are nominally adjusted to respond to the waves. It is accordingly an object of our invention to provide a new and improved method and means for calibrating such wide band pass circuits in such fashion.

The response of such wide band pass circuits is usually more or less non-uniform over the pass band, and is generall a maximum at some point in the pass band substantially removed from its center. It is, therefore, a further object of our invention to provide a new and improved method and means for calibrating such wide band pass circuits without reliance on their response at a single frequency.

The features of our invention which we believe to be novel are set forth with particularity in the appended claims. Our invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 schematically illustrates a radio receiver circuit embodying my invention, and Figs. 2 and 3 graphically show certain characteristics of the embodiment of Fig. 1.

In Fig. 1 the radio receiver includes an antenna suitable for picking up ultra high frequency radio waves, variably tuned radio frequency circuits l I, a radio frequency amplifier device l2, a frequency converter and intermediate frequency amplifier l3, a second detector and audio amplifier l4, an audio power amplifier device [5, and a loud speaker [5 through which audio signals are translated. In operation, the radio frequency circuits are variably tuned through the agency of a control knob ll, which is arranged to change the frequency to which the radio frequency circuits are tuned and the frequency to which the converter I3 is tuned simultaneously by equal increments, so that Waves from the antenna [0 are selected as desired by the radio frequency circuits I I, amplified to the radio frequency amplifier device i2, and converted in frequency to the intermediate frequency by the frequency converter I 3.

The amplifier device 12 has fiVe electrodes, of which the cathode I8 is connected through a resistance I!) ,to ground, and the anode 20 is connected serially through a load resistanceil and an anode current meter 22 to the positive terminal of a source of potential 23, whose negative terminal is connected to ground. The screen grid electrode 25 is connected through a resistance 26 to a point between the load resistance 2| and anode current meter 22, which point is bypassed to ground through a bypassing condenser 21. The suppressor electrode 28 is connected through a resistance 29 to ground. Waves selected by the radio frequency circuit from the Waves appearing on the antenna I 0 are amplified b the device l2 and appear across the load resistance 2|, from which they are impressed on the frequency converter and intermediate frequency amplifier l3 through a coupling condenser 30.

Because the radio frequency circuit I I and the local oscillator included in the frequency converter l3 are variable in frequency by adjustment of the control knob ll, they are not as stable in frequency as if the local oscillator in the frequency converter I3 were controlled in frequency by a quartz crystal. Even though the components of these circuits are made of high quality, selected for their high stability during temperature change and the like, frequency drift of the order of a few tenths of one percent of the radio frequency received is almost inevitable, and it is therefore desirable that the pass band of the radio frequenc circuit II and the intermediate frequency amplifier circuit l3 be of sufiicient width to allow for such frequency drift thereby to minimize the chance that such drift'shall cause the output Of the frequency converter i3 to change to a frequency outside the pass band of the intermediate frequency amplifier.

In Fig. 2 there is illustrated a desirable frequency characteristic for the receiver illustrated in Fig. 1. In this figure the carrier frequency is plotted as albscissa, and the output of the second detector I4 is plotted as ordinates. The curve 40 in this figure illustrates the relation between carrier frequency and the second detector output for one particular setting of the control knob I1. The curve 40 indicates that the receiver of Fig. 1 has a pass bandof about 300 kilocycles. It is desirable that this pass band indicated by the curve 40 be centered about the radio frequency for which the control knob I1 is nominally adjusted. Such frequency is indicated by the vertical line M in Fig. 2, which substantially bisects the pass band illustrated by the curve 40. i

The curve 40 has been illustrated as substantiallyflat topped, as is usual in radio receivers, the fiat topped characteristics being ordinarily produced by adjustment of interstage coupling in the intermediate frequency amplifier I3. It is usual that such adjustment should produce a substantially fiat topped curve All which, however, is not entirely uniform over the pass band. In the case illustrated, the central portion 42 is lower than the remaining part of the pass band, the low frequency part of the pass band 43 issomewhat higher, and the high frequency part of the pass band 44 provides maximum second detector output.

Calibration of. the receiver is accomplished through the medium of a calibration knob 50 which is effective to adjust the radio frequency circuit II and the frequency converter I3 in the same manner as the control knob [1, but in much smaller amounts.

An automatic volume control circuit is provided .for the receiver through a conductor 49 connected to a point onthe second detector I4 at which aunidirectional voltage is developed which varies in magnitude with the received signal intensity sufficiently for automatic volume control purposes, the conductor 49 being connected to supply such potential to the control electrodes 24 of the device I2 and to the control electrodes of the amplifier devices in the intermediate frequency amplifier I3 as a bias to control the gain thereof. The effect of the automatic volume control voltage on the conductor 49 upon the gain of the receiver may be observed byvariation in anode current of the discharge device I2 as indicated by the anode current meter 22. When a strong signal is received, the negative automatic volume control voltage increases, and the current indicated by the meter 22 decreases correspondingly.

The radio receiver of Fig. 1 may be calibrated by adjusting the calibrating knob 50 to a position such that the meter 22 reads a minimum, while a standard frequency signal is placed on the antenna l and the control knob I1 is set to that standard frequency. However, reference to Fig. 2 indicates that the receiver is then so adjusted that the standard frequency applied to the antenna It) lies within the pass band illustrated by the curve 40 at the point 44. Such adjustment is satisfactory if drift of the pass band is toward higher frequencies, or if the signal drift is toward lower frequencies, since in either such case signal transmission through the receiver is not affected until the pass band or signal frequency has changed in frequency by about 300 kilocycles. It is quite possible, however, that the pass band illustrated by the curve 40 may move to lower frequencies, or that the signal may drift to higher frequencies, and in such case transmission through the receiver is affected after only a slight drift. Calibration of the control knob I1 in dependence on a single standard frequency is therefore undesirable, and generally does not center the pass band 49 about the nominal frequency, --as illustrated by the vertical line 4| in Fig. 2.

Certain remaining portions of the receiver are.

utilized in the calibration of the control knob I1, and include the audio power amplifier I5, a gang switch including switch members 5! through 56, a tuned circuit 51, a quartz crystal oscillator 58, and a tuned rectifier circuit 59. When the re ceiver' is operating normally the conductor 49 is connected to the control electrode 24 of device I2 and the control electrode of the electron discharge devices in the amplifier i3 through the switch member 55. The cathode 69 of the power amplifier I5 is connected to ground, and its control electrode BI is connected to ground through a resistance 52, and through a coupling condenser 63 and a switch member 52 to the output of the second detector and audio amplifier I4. The screen grid electrode 64 of the power amplifier I5 is connected through a resistance 65 to the positive terminal of the source 23, and the anode 66 of the power amplifier I5 is connected through the switch member 5i and through the primary of an output transformer 6'! to the same positive terminal of the source 23. The loud speaker I6 is connected to the secondary of the transformer 61.

The switch means including the switch members 5! through 56 is provided for the purpose of changing certain connections in the receiver to provide for the proper calibration of the control knob I'I. Connections through theswitch members El, 52 and 56 are as described when the switch means is in its right-hand position, the radio frequency circuit II being connected to the antenna I0 through the switch member 54 in that position. The switch members 53 and 55 make no connection in their right-hand position.

When the switch means is moved to its lefthand position to provide for calibration of the control knob H, the anode 65 of the power amplifier I5 is disconnected from the transformer 61 and connected through the switch member 5| 5 to one terminal of the tuned circuit 51. The coupling condenser 63 is disconnected from the output of the audio amplifier I4 and is connected through the switch member 52 to a conductor 68 which is coupled through a coupling condenser 69 .155 to the other terminal of the tuned circuit 51. An intermediate tap of the inductance of the tuned circuit 51 is connected to the positive terminal of the source 23 so as to provide a path for operating'current to the anode 66 of the power 0 amplifier I5. The connections described provide for the operation of the power amplifier I5 with the tuned circuit 5'! as an oscillator whose output appears on the conductor 68.

When the switch member 53 is in its left-hand 65 position, the conductor 68 is connected to the suppressor electrode 28 so that the output of the above described oscillator is impressed on that suppressor electrode to modulate any ultra high frequency wave appearing 'on the control elec- ='7 0 trode 24. Member 54, in its left-hand position, connects the radio frequency circuit I I to the out put of the crystal oscillator 58. Switch member 55, in its left-hand position, connects the crystal oscillator 58 to the positive terminal of the source "75 2.3 of potential so as to. supply operating current to the crystal oscillator. Member 56, in its lefthand position, disconnects the automatic Volume control conductor 49 from the control electrode of the device I2 and the discharge devices in the intermediate frequency amplifier I3, and connects such control electrodes through the switch member as and through a resistance 8.0 shunted by a condenser 8! to ground.

With such connections made through the switch members 5i through 56, the oscillator in cluding the power amplifier I5 and tuned circuit 5'! is adjusted to operate at a frequency which is half the frequency between points on the curve 48 of Fig. 2 somewhat removed in either direction from the substantially flat topped portion thereof. In the case illustrated in Fig. 2 this frequency should be about 175 kilocycles and should be adjusted experimentally as will be explained hereinafter. The output frequency of the crystal oscillator 58 must be a frequency which is nominally marked on the control knob I1, to which frequency the con rol knob ll must be set during all calibration operation. The ultra high frequency wave from the oscillator 58 is transmitted through the radio frequency circuit I I to the amplifier device I2, where it is modulated in amplitude by the 175 kilocycle wave from the oscillator including tuned circuit 5'! and power amplifier device I5, such wave being impressed on the suppressor electrode 25. The output of the amplifier device I2 is transmitted through the frequency converter and intermediate frequency amplifier I3 and through the detector It. where the modulation of the ultra high frequency wave from the crystal oscillator 58 is detected.

The output of the detector It appears on the conductor 82, and in the calibrationoperation comprises a 1'75 kilocycle wave corresponding to the wave from the conductor is which modulatee the ultra high frequency wave in the device 52. The conductor 82 is connected through a coupling condenser 83, an inductance 8- 3 and a resistor 85, all arranged in series, to the ungrounded terminal of the resistance 80. A bypassing condenser 85 is connected between ground and the point between the inductance 84 and resistance 85. Adjustment of either the condenser 85 or the inductance 224 may be utilized to make the combination of condenser 83 and inductance 84 resonant, in series, at the frequency of the waves on conductor 68. Whenever the operating frequency of the oscillator including the power amplifier I5 and tuned circuit 51 is adjusted, the tuned circuit including the condenser 83 and inductance 34 must be readjusted in correspondence. Voltage appearing across the inductance 84 is rectified in a diode rectifier 8? whose anode is connected to a point between the condenser 83 and inductance t4 and whose cathode is connected through a resistance 88 to ground and through a second resistance 89 to the positive terminal of the source 23 of potential. A bypassing condenser 90 is connected in shunt to the resistance 88 to bypass alternating current therearound and to maintain the cathode of the diode 8'! effectively at ground potential for such alternating current.

Since the cathode of the diode 8'! is maintained slightly positive with respect to ground by the voltage divider including resistance 83 and 89, which are connected in shunt to the source 23 of potential, the diode 81 cannot rectify until the alternating voltage across the inductance 84 is above a certain minimum level. The output of the oscillator including the power amplifier I5 and tuned circuit 51 must be suflicient to cause the detector I4 to impress a sufficiently high voltage upon conductor 82 to cause the diode 81 to rectify. By preventing the diode rectifier from responding to voltages below this predetermined minimum level, the effect of extraneous noise voltages and the like on the tuned rectifier is minimized.

When voltage appears on the conductor 82 in sufiicient intensity to cause the diode ill! to rectify, the rectified output voltage appears across the resistance to, and is impressed upon the control electrode 26 of the device I 2 and the control electrode of the intermediate frequency amplifiers I3 through a conductor 9| and switch member 56 so as to reduce the gain of the device I 2 and the inermediate frequency amplifier I3 as the voltage on the conductor 32 increases. When the gain of these devices is reduced, the anode current of the device I2, as indicated by the meter 22, is correspondingly reduced, as when the automatic volume control voltage 5! is applied to the control electrodes of these devices and increases.

With the receiver so arranged and connected, accurate calibration of the control knob I! is possible so that the nominal frequency indicated by the control knob I! is located in the center of the pass band. of the receiver. To accomplish this calibration with the control knob ll set at a nominal frequency equal to the standard frequency of the crystal oscillator 55, the calibration knob 58 is adjusted and results are obtained as indicated in Fig. 3. In this figure the adjustment of the calibration knob o l is plotted as abscissa and the anode current of the device I2, as indicated by the meter 22, is plotted as ordinates. The curve lull illustrates the relation between these quantities, the portion Iii! showing that the anode current as indicated by the meter 22 is a maximum when the calibration knob 59 is in one extreme position. As the calibration knob is turned from that extreme position toward the other extreme position. the anode current of the device I2 decreases very slightly for a time along curve portion IilI, then decreases more rapidly along curve portion i 632 to a minimum at point Hi3. Further angular rotation of the calibration knob 50 from this minimum produces an increase in anode current along curve portion let to a maximum point l @5, and another drop along curve portion its to a minimum, at a point I01, which second minimum is usually of a different value from the first minimum at point I03. Fur.- ther rotation of the calibration knob 59 produces a rise in anode current along portion I88 of the curve I50, and a succeeding gradual rise along the portion I99 of the curve I68 to the other extreme position of the knob 50.

The point I55 of the curve I00, at which the anode current is maximum, is the correct setting of the knob 58. The amount of anode current indicated by point I05 and the degree of curvature of curve H30 near point H15 may be adjusted within limits by suitable adjustment of the frequency of operation of tuned circuit 51. It is desirable that the degree of curvature of curve I fill near point I 65 shall be reasonably great to aid in accu-- rate adjustment of calibration knob 50.

When a radio receiver, or other variably tuned apparatus, is adjusted in accordance with my invention so that the nominal adjustment is exactly at the center of the pass band of the apparatus, frequency drift of the apparatus, or of signals transmitted therethrough, has a minimum chance for reducing the signal transmission through the apparatus, since the amount of drift possible without ill result is substantially the same in either direction. In order to achieve these results, it is desirable that the modulation component of the output of detector I4 be measured rather than the continuous current component corresponding to carrier wave intensity. In the apparatus illustrated the modulation component corresponds to the alternating potential wave from the oscillator including power amplifier l5 and tuned circuit 1. It is also desirable that the frequency of this modulating wave be so adjusted as to produce a dip in output corresponding to the slight increase in anode current at the portion I85 of the curve Hi9, when the standard frequency carrier wave carrying the modulation wave is centered in the pass band of the apparatus. By this arrangement we have provided an ultra high frequency receiver which is variably tuned, and which has minimum change of drifting so far in frequency as to fail to respond to a signal to which the control knob ll 7 is nominally tuned.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from our invention in its broader aspects, and we, therefore, aim in' the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, apparatusfor translating an ultra high frequency wave having side bands within a predetermined frequency range, said apparatus including a variably tuned circuit, means for calibrating the variable tuning of said circuit, said apparatus having a pass band substantially Wider than said predetermined frequency range to 'allow for drift in frequency of said pass band or of said wave so as to maintain translation of said Wave during such drift, means for impressing a standard frequency wave on said apparatus and for producing side bands associatedwith said wave and spaced apart by an amount comparable to the pass band of said apparatus, and means for indicating the amount of said side bands transmitted through said apparatus whereby said calibrating means may be adjusted to produce a characteristic amount of said side bands-in the output of said apparatus when said standard frequency is centered in said pass band.

2. In combination, a superheterodyne receiver adapted to receive a high frequency wave having side bands within a predetermined frequency range, said receiver having a pass band substantially wider than said range to allow for drift in frequency of said pass band or of said Wave so as to maintain reception of said wave during said drift, means for impressing a standard frequency wave on said receiver and for producing side bands associated with said wave and spaced apart by an amount comparable to the pass band of said receiver, and means for indicating the amount of said side bands transmitted through said receiver, whereby said receiver may be calibrated in accordance with the transmission of a characteristic amount of said side bands through said receiver so that said standard frequencywave is received at the center of said pass band.

3. In combination, a superheterodyne receiver having means including a local oscillator for converting the frequency of a received wave to an intermediate frequency, the frequency of such received wave and the operating frequency of said oscillator being subject to drift in frequency, said received wave having side bands within a predetermined frequency range and said receiver having a pass band substantially wider than said predetermined frequency range to allow for drift in frequency of said wave or of the operating frequency of said oscillator so as to maintain reception of said wave during such drift, means for impressing a standard frequency wave on said receiver and for producing side bands associated with said wave and spaced apart by an amount comparable to the pass band of said receiver, and means for indicating the amount'of said side bands received by said receiver, whereby said receiver may be calibrated in accordance with the transmissionof a characteristic amount of said side bands through said receiver so that said standard frequency wave is received at the center of said pass band.

PAUL C. GARDINER. JOHN E. MAYNARD. 

